v 5.2

⟨ E_{Σ} ⟩^{2} = ⟨ E_{1} ⟩^{2} + ⟨ E_{2} ⟩^{2} + 2 ⟨ E_{1} E_{2} ⟩

${\langle {\bf E}_\Sigma \rangle^2 = \langle {\bf E}_1 \rangle^2 + \langle {\bf E}_2 \rangle^2 + 2 \langle {\bf E}_1 {\bf E}_2 \rangle}$

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